1. Field of the Invention
This invention relates to a force transducer which converts a force into an electrical signal using the piezoresistive effect of a semiconductor and also to a pressure detecting circuit using the force transducer, and more particularly to such device and circuit which produces a voltage output corresponding to a force which is applied perpendicularly to the crystal face of a silicon semiconductor while a current flows in the silicon semiconductor.
2. Description of the Related Art
Force transducers are widely used in various fields as force detecting sensors and are therefore requested to have such an ability as to measure a force precisely with no influence by the environment.
On many occasions force transducers are used in a very severe environment. In recent years, force transducers have been used under high temperature environment for measuring the pressure in an engine cylinder in order to control the ignition timing of the engine according to the measured pressure.
Force transducers used under such circumstances are required to measure steadily the pressure of combustion gas pressure in the engine with good responsiveness in a wide range of the temperature environment.
FIGS. 16A and 16B show an example of a prior art force transducer (U.S. Pat. No. 4,833,929); FIG. 16A is a schematic plan view of the sensor, and FIG. 16B is a schematic side view of the sensor.
This prior art combustion pressure sensor comprises a rectangular planar p-type silicon semiconductor 10 (having a resistivity of 8 .OMEGA.cm and a thickness of about 200 .OMEGA.m) with a crystal face (110) 10a on which a force W is to be applied, a force transmission block 30 connected to the crystal face 10a of the silicon semiconductor 10 for transmitting the force W perpendicularly to the crystal face 10a , and a support bed 20 connected to the other face of the silicon semiconductor 10. The force transducer is constructed in such a manner that a combustion gas pressure P in a non-illustrated engine cylinder acts as a force perpendicularly on the top face 30a of the force transmission block 30 via a diaphragm. At that time, the force W can be obtained by the equation W =PXA where A is a pressure receiving area of the diaphragm.
A pair of confronting input electrodes 14, 14' are formed on the silicon semiconductor 10 at an angle of 45 degrees going away anticlockwise from the direction of &lt;110&gt; of the crystal. A current flows from a power supply to the silicon semiconductor 10 via the input electrodes 14, 14'.
Similarly, a pair of confronting output electrodes 12, 12' is formed on the silicon semiconductor 10 at an angle 45 degrees going away anticlockwise from the direction of &lt;001&gt; of the crystal. When a force W is applied perpendicularly to the crystal face 10a of the silicon semiconductor 10, the output electrodes 12, 12' output a voltage corresponding to the force W by the piezoresistive effect of the silicon semiconductor. It is therefore possible to measure the force W and hence the combustion gas pressure P (hereinafter called "combustion pressure) by measuring the output voltage.
Specifically, when a force W is applied on the crystal face 10a of the silicon semiconductor 10 via the force transmission block 30, a voltage V.sub.sens to be outputted from the output electrodes 12, 12' is a superposed voltage of a so-called offset voltage V.sub.off and a voltage output .DELTA.V.sub.OLD which is given by Equation 1: EQU .DELTA.V.sub.OLD =I.times.R.times..pi..sub.63 '.times..sigma..sub.Z =V.times..pi..sub.63 '.times..sigma..sub.Z [Equation 1]
where I is the current (A) flowing in the silicon semiconductor, R is the resistance (.OMEGA.) between the input electrodes, V is the voltage (V) impressed to the silicon semiconductor, .pi.63' is the piezoresistive coefficient (cm.sup.2 /kg) of the silicon semiconductor of FIGS. 16A and 16B, and .sigma..sub.Z is the compressive stress generated on the crystal face 10a .
The piezoresistive coefficient .pi..sub.63 ' in Equation 1 may alternatively be expressed by Equation 2 as disclosed in "Use of Piezoresistive Materials in the Measurement of Displacement, Force and Torque" by R. N. Thurston, THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol. 29, No. 10, October 1957. For example, in the case of a p-type silicon crystal having a resistivity of about 8 .OMEGA.cm, the piezoresistive coefficient .pi..sub.63 ' can be calculated as about -33X10-.sup.6 cm.sup.2 /kg. ##EQU1##
In Equation 2 , .pi..sub.11, .pi..sub.12 and .pi..sub.44 are the piezoresistive coefficients of a cubic crystal. For example, in the case of a p-type silicon crystal having a resistivity of about 8 .OMEGA.cm, .pi..sub.11 32 6.times.10.sup.-6 cm.sup.2 /kg, .pi..sub.12 =-1.times.10.sup.-6 cm.sup.2 /kg and .pi..sub.44 =138.times.10.sup.-6 cm.sup.2 /kg.
Accordingly, given that a housing having the diaphragm is equipped with, as a pressure detecting means, a force transducer 2000 of FIGS. 16A and 16B having a silicon semiconductor 10, which is known as a highly elastic material, it is possible to realize a combustion pressure sensor which is also able to measure a static pressure.
However, the prior force transducer 2000 has the following problems.
First Problem
The prior force transducer 2000 requires a pair of input electrodes 14, 14' and a pair of output electrodes 12, 12', both pair to be mounted on the silicon semiconductor 10. If the force transducer 2000 is to be assembled in a housing to constitute a combustion pressure sensor, it requires four lead wires to be connected to the respective electrodes 12, 12' , 14, 14'.
The combustion pressure sensor, whose object is to detect the pressure-in-cylinder information more accurately, is mounted directly on an engine. It is a common knowledge that if it is mounted on the engine, the combustion pressure sensor should be left unprotected in a severe environment which has some fear that the sensor might be influenced such as by heat, vibration and external turbulent magnetic field. It can be said that the degree of reliability at the joints which tend to receive heat, vibration, etc. is proportional to the number of lead wires. With this arrangement, because four lead wires and hence many lead wires are used, high-reliability of the joints and low cost of production cannot be achieved. Consequently a force transducer which is simple in structure with the lead wires reduced in number in an effort to secure the high reliability and low cost of production has been desired.
Second Problem
In the prior force transducer 2000, since its output voltage is small, the voltage output tends to be influenced by external turbulent magnetic field if it is used as a combustion pressure sensor. Therefore a force transducer which is able to output a higher voltage has been looked for.
Third Problem
If the combustion pressure sensor is mounted on an engine, the force transducer 2000 assembled in the combustion pressure sensor is left unprotected in a high-temperature environment. More particularly, if the engine is operated at a high r.p.m., the temperature of the force transducer 2000 would often reached to 150.degree. C. or higher. However it is also common knowledge that the silicon semiconductor 10 to be used in the force transducer 2000 is made of a highly elastic material but its resistivity and piezoresistive effect are highly dependent on temperature.
Especially the silicon semiconductor 10 constituting the force transducer 2000 of FIGS. 16A and 16B has a high resistivity of about 8 .OMEGA.cm (i.e., a low impurity concentration) and is remarkably dependent on temperature; the temperature dependency is about 0.8%/.degree.C. in resistivity and about -0.25%/.degree.C. in piezoresistive effect. The resistivity and the piezoresistive effect here are equivalent to the inter-input-electrode resistance R (hereinafter called "the input resistance") and the piezoresistive effect .pi..sub.63 ' of Equation 1 . In the prior force transducer 2000, since these values are highly dependent on temperature, the voltage .DELTA.V.sub.OLD varies with temperature so that precise detection of the combustion gas pressure P cannot be achieved.
Fourth Problem
The prior force transducer 2000 is encountered with a phenomenon that the input resistance is sharply dropped in a range of 150.degree. C. to 200.degree. C., which results from a physical characteristic that the silicon semiconductor 10 having a resistivity of about 8 .OMEGA.cm possesses in nature.
Consequently, with the prior combustion pressure sensor employing this force transducer 2000, it is absolutely impossible to measure the combustion pressure P of the engine under the high-temperature and high-pressure operating conditions in which the temperature exceeds 200.degree. C., and so it has been the desired object to extend the usable temperature range of the sensor to cover higher temperatures.